AT&T on Tuesday gave a list of criteria for upgrading to Apple's 3G iPhone that goes on sale July 11.

The exclusive wireless carrier for the iPhone in the United States said customers using the smartphone today are automatically eligible for an upgrade to the new iPhone at the subsidized price of $199 for the 8-GB model and $299 for 16-GB version, AT&T spokesman Mark Siegel told InformationWeek. As in all upgrades at the subsidized price, customers have to sign a new two-year contract no matter where they are in their current contract.

If a person bought an iPhone on or after May 27, then they get to upgrade at no additional charge, and get a refund for the difference between the price of the old iPhone and the subsidized 3G model, minus a 10% restocking fee.

For AT&T customers using something other than an iPhone, the upgrade process is trickier. In order to be eligible for the subsidized price, customers have to first meet AT&T's credit criteria. Siegel declined to give details, saying that people are judged on an individual basis. "It really varies from person to person," Siegel said. "There's a whole lot of variables, and I would hate to give specific criteria."

Payment history is certainly one factor, but customers will have to call AT&T, go to one of its stores, or check online to know for sure whether they're creditworthy, as defined by AT&T. Also weighing into the decision is how long the person has left on the current contract. Again, that's also a slippery slope, since it's tied to the person's credit standing. "It's not a one-size-fits-all approach," Siegel said.

People who are ineligible for the subsidized price can certainly buy an iPhone for the "unsubsidized price," Siegel said. What that is, however, has yet to be determined. AT&T will release that price sometime before the iPhone goes on sale.

One thing for certain, the new iPhone will require a minimum $30-a-month data plan, versus $20 for the original model, for the length of the two-year contract. Consumer electronics retailer Best Buy has posted a general list of criteria for AT&T cell phone upgrades.

Apple chief executive Steve Jobs introduced the 3G iPhone June 9 at the company's Worldwide Developers Conference in San Francisco. Unlike the original iPhone, the new model supports carriers' faster 3G, or third generation, data networks. The 3G iPhone will be available in 22 countries, versus six for the original, when it ships. Apple plans to make the smartphone available in 70 countries by the end of the year.